Remote Sensing Finds Water Loss on Ryugu

Jan 10, 2021 | Asteroids, Daily Space, Spacecraft

IMAGE: Japan’s Hayabusa2 spacecraft snapped pictures of the asteroid Ryugu while flying alongside it two years ago. The spacecraft later returned rock samples from the asteroid to Earth. CREDIT: JAXA

From the very big planetary-like body to the very small planetary-like body, we go now to new research coming out about Ryugu, our second favorite asteroid this last year. While analysis of the returned sample is currently underway, scientists are already working on data received from the spacecraft’s instruments to learn more about the history of the asteroid.

In a new paper published in Nature Astronomy, researchers examine why Ryugu seems to contain fewer water-bearing minerals than other asteroids we’ve studied. Since asteroids seem to be the broken up bits of larger bodies, in most cases, this lack of water must go directly back to the parent body of Ryugu.

Study co-author Ralph Millliken explains: One of the things we’re trying to understand is the distribution of water in the early solar system, and how that water may have been delivered to Earth. Water-bearing asteroids are thought to have played a role in that, so by studying Ryugu up close and returning samples from it, we can better understand the abundance and history of water-bearing minerals on these kinds of asteroids.

In fact, one of the reasons Ryugu was chosen as a sampling target for Hayabusa2 was the assumption that Ryugu was a particular type of asteroid that had water-bearing minerals and organic compounds; the kind of asteroid that is the parent body for meteorites bearing carbonaceous chondrites that brought water to Earth.

Instead, Ryugu seems to have lost its water at some point in its history. Perhaps the parent body dried out. Perhaps some sort of catastrophic disruption blew Ryugu apart, and it re-formed as the rubble pile asteroid it is today, but its water evaporated during the event. Or maybe Ryugu had some close calls with the Sun and dried up that way.

Now remember how Hayabusa2 took a shot at Ryugu, blowing a small crater into its surface to get underneath? Turns out that it’s dry there, too. This means it’s not likely to be that Sun-based hypothesis. Milliken goes on to explain: You’d expect high-temperature heating from the sun to happen mostly at the surface and not penetrate too far into the subsurface. But what we see is that the surface and subsurface are pretty similar and both are relatively poor in water, which brings us back to the idea that it was Ryugu’s parent body that had been altered.

One possible answer down; at least two more to go. Of course, everything could change once the physical samples are analyzed in the lab. Remote sensing is great, but the hands-on aspects of science can bring everything into focus and answer the question of just how Ryugu lost its water. And maybe even help us understand how Earth got its water.

More Information

Brown University press release 

Thermally altered subsurface material of asteroid (162173) Ryugu,” K. Kitazato, R. E. Milliken, and Y. Tsuda, 2021 January 4, Nature Astronomy


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